Fiqh-us Sunnah

Fiqh-us-Sunnah Volume 003, Zakaat and Fasting, Fiqh 3.137.

If someone has food in his mouth when fajr is beginning, he should spit it out. If he is having intercourse (with his wife) at that time, he should immediately stop. If he does so, his fast will still be valid. If he continues in these actions at that time, he will have broken his fast. Al-Bukhari and Muslim record from Aishah that the Prophet said: "Bilal makes the call to prayer while it is still night; therefore, eat and drink until Ibn Umm Maktum makes the call to prayer."